1. Field of the Invention
The present invention relates to a low-complexity Maximum Likelihood (ML) detection apparatus and method in a wireless communication system.
2. Description of the Related Art
Many studies have recently been conducted on techniques for reducing complexity, a problem encountered with an ML receiver, while satisfying the performance requirement of the ML receiver. In this context, ML receivers with far less complexity, viable for a real system, have been proposed, such as QR-decomposition Modified Maximum Likelihood Detector (QRM-MLD), Recursive Modified Maximum Likelihood (RMML), Sorted Modified Maximum Likelihood (SMML), and Minimum Mean Square Error Ordered Successive Interference Cancellation (MMSE-OSIC) with Candidates (MOC).
Despite significant progress that has been achieved in reducing the complexity of ML receivers, the complexity reduction for low-complexity ML receivers is based on the premise that all streams are modulated by the same scheme.
Conventionally, most low-complexity ML receivers use ordering/sorting, or perform ordering/sorting using the norms of the column vectors of a channel matrix since the norm of each column vector is proportional to Signal-to-Noise Ratio (SNR), which can be ordered according to norms. Hence, if the same modulation scheme applies to all streams, a higher column vector norm increases the SNR, thereby decreasing error probability.
However, if a different modulation scheme is used for each stream, a high error probability may result depending on the used modulation scheme even though the SNR is high. In this case, column vector norm-based ordering/sorting alone makes it impossible to order/sort error probabilities. As a consequence, the error probability performance of a conventional low-complexity ML receiver can be degraded.